Heat pumps are a device for the heating and cooling of an indoor space during winter and summer months, respectively. Generally speaking, heat pumps have a compressor and two heat exchange coils coupled with the compressor via suitable valving. One of the coils is indoors and the other is outdoors. During summer cooling of the indoor space, the indoor coil is operated as an evaporator for liquid refrigerant while the outdoor coil is operated as a condenser for refrigerant vapors. For heating the space during winter months, the indoor coil is operated as a condenser and the outdoor coil is operated as an evaporator.
In temperate climate zones, the winter heating load is usually greater than the summer cooling load. In order to effect the desired heating of the indoor space the compressor of the heat pump must be a relatively large, high horsepower compressor fitted to the heating demand or a supplementary heating system must be provided. To avoid the necessity for large and expensive high horsepower compressors, supplementary heating systems have heretofore been used.
Normally, electric resistance heaters built into in indoor (winter time) condenser were employed as supplementary heat sources. The resistance heater was operated whenever additional heat was required due to low outdoor temperatures. Up to now, such a construction has generally been satisfactory. However, with increasing energy costs and in particular with the prospect of having to pay peak demand penalties for electric energy, such resistance heaters are likely to become economically unfeasible, particularly since such heaters have an essentially zero diversity factor, that is they cannot be turned off during peak electricity demand times, (e.g. during the evening hours) and turned on during low demand times (e.g. during night time).
The alternative of using fuel, e.g. fossil fuel fired supplementary heaters is not readily realized because it is virtually impossible to obtain soot-free combustion, particularly with compact, high heat release burners such as would be required for incorporation in the indoor condenser of the heat pump. To insure a clean indoor atmosphere, it would therefore be mandatory that the combustion gases of the supplementary fuel fired heater be exhausted to the outdoor atmosphere. When so constructed, however, there is insufficient space for incorporating the supplementary heater in the condenser, particularly in a manner which would not interfere with the normal operation of the heat pump in general and the condenser in particular. To install the auxiliary fuel fired heater externally, however, sets it apart from the heat pump, requires separate controls, ducting, etc. and renders the installation as well as the operation and maintenance of the supplementary heater expensive, wasteful of space and therefore undesirable.